Infectious diseases, such as influenza, lead to high morbidity and mortality in elderly populations. In addition, the efficacy of vaccines is also significantly reduced for elderly populations, leaving them much more vulnerable to infection. While it is well known that the adaptive immune response to influenza infection and immunization declines with aging, the impact of specific age-related changes in T cell function remains to be elucidated. Defining the underlying defects in the immune response with aging in human populations is extremely difficult. Fortunately, mouse models allow us to precisely examine age-related changes in the immune system and determine the effect of these changes on a response to a particular pathogen. Thus, the key focus of this program is to assess the development of age-related changes in T cell function, define the mechanisms responsible for these defects and determine if they are also involved in declines in the human immune system. Project 1 Impact of aging on CD4 immunity to flu will examine the impact of age on CD4 T cell primary and memory responses and if this can be enhanced. Project 2 Influence of aging on T follicular helper (Tfh) cells will focus on examining the role of age-related changes in CD4 T cel cognate helper function for humoral responses and how this impacts the production of protective antibodies following vaccination. Project 3 Impact of age on CD8+ T cell immunity to respiratory infection will examine CDS T cell memory generation and function, which is dramatically reduced with aging possibly due to changes in homeostasis of memory T cell subsets. Project 4 Impact of aging on the T cell repertoire and cellular immunity to influenza virus will examine age- related changes in CD4 and CDS T cell repertoire and how these influence the ability to respond to influenza infection. The knowledge generated will allow the future development of strategies to overcome these defects and enhance vaccine efficacy for the elderly. Project 5 Impact of aging on T cell responses to influenza vaccination will translat findings in mouse models to studies in human naive and memory T cells from different age groups of vaccinated adults.